Computate an under-approximation to the set of all facts with unique
instances, i.e., fact whose instances never occur more than once in a
state. We use this information to reason about protocols that exploit
exclusivity of linear facts.

Computing injective fact instances.

Compute a simple under-approximation to the set of facts with injective
instances. A fact-tag is has injective instances, if there is no state of
the protocol with more than one instance with the same term as a first
argument of the fact-tag.

We compute the under-approximation by checking that
(1) the fact-tag is linear,
(2) every introduction of such a fact-tag is protected by a Fr-fact of the
first term, and
(3) every rule has at most one copy of this fact-tag in the conlcusion and
the first term arguments agree.

We exclude facts that are not copied in a rule, as they are already handled
properly by the naive backwards reasoning.